The present invention relates to the removal of compounds from petroleum refinery streams which foul process equipment. More specifically, it relates to a process for separating stable polycyclic aromatic compounds which form during the hydrocracking process and which foul downstream process equipment by scaling and plugging flow in and around such downstream equipment.
Petroleum refinery hydrocracking processes are well known and developed. Such processes upgrade mixtures of hydrocarbons to supply more valuable product streams.
Hydrocracking is a high severity hydrotreating operation in which high molecular weight compounds are cracked to lower boiling materials. Severity is increased by operating at higher temperature and longer contact time than in hydrotreating. Increased hydrogen pressure controls deposits and catalyst fouling. Unlike thermal or catalytic cracking, hydrocracking decreases the molecular weight of aromatic compounds and fills a specific need for processing streams high in aromatic material, such as cycle stocks from catalytic or thermal cracking, coker products, or coal liquids. For example, catalytic cycle stock can be cracked to a naphtha fraction that is an excellent feed for catalytic reforming to make premium-octane gasoline or petrochemical aromatic material.
Hydrocracking us used extensively on distillate stocks. The hydrocracking process is applied to refinery stocks for premium-quality kerosene and diesel or jet fuels low in sulfur and nitrogen. The light products from hydrocracking are also rich in isobutane, an important raw material for alkylation.
Hydrocracking is of increasing importance in view of the trend to heavier crudes and the need for processing synthetic crudes. Thus, hydrocracking of residuum, tar sands, and shale oil of 10-11% hydrogen content may be more attractive than upgrading coil liquids with only 6% hydrogen and high aromatic content.
In recent times, as the worldwide supply of light, sweet crude oil for refinery feedstock has become more scarce, there has been a significant trend toward conversion of higher boiling compounds to lower boiling ones. This "bottom of the barrel" or "hard processing" has increased potential downstream fouling problems by tending to create even greater quantities of heavier, converted cyclic compounds, such as polycyclic aromatics, in the initial stages of the refining process. The addition of a hydrocracking reactor, and such process units as residual desulfurization units, makes the need for an economic solution to the fouling problem even more desired.
As demand for distillate fuels increased, refiners installed hydrocrackers to convert Vacuum Gas Oil (VGO) to jet and diesel. Catalysts were developed that exhibited excellent distillate selectively, high conversion activity and stability for heavier feedstocks.
A trend in recent years in the push for higher yielding reactors has been the use of longer life catalysts having an increasing amount of molecular sieve. A well known class of catalysts with a higher degree of molecular sieve are the "zeolite" type catalysts. One result of the zeolitic catalyst in hydrocracking reactors is the formation of aromatic compounds, which in turn once again increases the presence of compounds having a propensity to form stable polycyclic aromatic compounds. Additionally, these stable polycyclic aromatic compounds contribute to catalyst fouling and cooling. The formation of stable polycyclic aromatic dimer compounds has been found to increase during "end of run" conditions just prior to catalyst replacement, when hydrocracker temperatures may approach 850.degree.-900.degree. F. Thus, an efficient and economical improved process for the removal of stable polycyclic aromatic compounds is much desired as a means for reducing fouling of refinery process equipment and catalyst coking.
In addition to high conversion distillate production, another trend in the 1980's has been to send unconverted fractionator bottoms from the hydrocracker to units such as FCC units, ethylene crackers and lube plants which benefit from highly paraffinic feedstocks. The VGO-range bottoms material is desulfurized, denitrified and highly saturated during its residence time in the hydrocracker.
U.S. Pat. No. 3,619,407 issued on Nov. 9, 1971 to Hendricks et al. describes one hydrocracking catalyst for use in a hydrocracking process, and is further relevant in describing certain aspects of the problem which is addressed by the present invention. The reference discloses the problem of the formation of polycyclic aromatic compounds which are identified in the reference as being benzocorenene. The reference describes the known tendency for such compounds to "plate out" onto cooler downstream equipment such as heat exchanger surfaces. The claimed solution described in the reference is the withdrawal or "bleeding" of a portion of the hydrocracker effluent, in order to reduce the concentration of polycyclic aromatics existing in such effluent.
U.S. Pat. No. 4,447,315 issued on May 8, 1984 to Lamb et al. is considered relevant for disclosing a process scheme for reducing the concentration of polynuclear aromatic compounds, or "PNA's" in a hydrocracking process by separating hydrocracker effluent in a fractionator, and contacting the fractionator bottoms in an adsorption unit with an adsorbent which selectively retains the PNA compounds, and recycling the fractionator bottoms back to the hydrocracking reactor.
U.S. Pat. No. 4,655,903 issued on Apr. 7, 1989 to Rahbe et al. discloses a method of upgrading residuals by removing unstable polynuclear hydrocarbons known to be coke precursors by mixing with the residual a light hydrocarbon solvent, and separating polynuclear hydrocarbons from the unconverted residual.